In high performance jet turbine engines for aircraft, it is frequently desirable to provide a variable area exhaust nozzle to improve engine performance. For example, two dimensional (2D) nozzles employ two generally stationary side walls and upper and lower flaps which define the boundaries of the exhaust opening. By appropriate actuation means, the upper and lower flaps may be opened or closed to change the area of the exhaust opening. In so moving, the upper and lower flaps slide relative to the side walls.
Space between the moveable flaps and the stationary side walls provides a leakage path for relatively high pressure gases in the main exhaust flow stream. A major concern in the design of such exhaust systems, therefore, is the control of leakage through such space or gap.
Two types of exhaust seals that have been used in the past are elastic leaf seals and hinge leaf seals. Elastic leaf seals consist of a generally "U" shaped member which is fastened on one side to one of the members to be sealed and is in rubbing contact on its other side to the other member. This seal may be spring loaded so that as the two members move apart it maintains contact therebetween. As with other seals, two basic requirements must be satisfied. Namely, it must be strong enough to withstand the relatively high pressure of the exhaust flow stream, and it must, at the same time, be flexible enough to expand and contract with variations in the gap. In general, elastic leaf seals are unable to satisfy these requirements. That is, if they are strong enough to stand the pressure, they may not be flexible enough to accomodate the required expansion.
Hinge leaf seals generally include two flat sheets which pivot from a common hinge point. One of the sheets is attached to one of the members to be sealed and the other sheet is in rubbing contact with the other member. Such seals satisfy the requirements of high strength and ability to expand and contract as the gap opens and closes. However, a problem which exists in such seals is that a leakage path for high pressure gases exists through the hinge. In spite of careful design, the performance penalty in current nozzles is in the range of 0.6% to 0.9% in flow and thrust.